The idea of prototyping to uncover 'unknown unknowns' resonates with me, it's like a reconnaissance mission before the actual project. This could indeed save a lot of time and effort in the long run...
It helps if you have tests because when prototyping, you can write things in a way which break tests and ideally break other features. It works enough that you can validate the one idea. That way it will never pass CI.
Besides discovering unknown-unknowns it also helps in conveying design/idea to rest of the team.
Which actually doesn't work so well in AoE. The game doesn't have a concept of army supplies, so you usually just want a huge unstoppable army you can send anywhere.